The rendering product of Category 2 and 3 Animal By-Products is known as sterilized mass (SM) and it is mainly composed of fat and proteins, making it interesting substrate for anaerobic digestion. Batch and semi-continuous laboratory experiments were carried out to investigate the effect of SM addition in co-digestion with sewage sludge on methane production and possible process limitations. Results showed that SM addition in the feed mixture up to 5% (w/w), corresponding to 68.1% of the organic loading, increased methane production 5.7 times, without any indication of process inhibition. Further increase of SM addition at 7.5% (w/w) caused methane production decrease and volatile solids removal reduction, that was mainly related to remarkably increased free ammonia concentration in the digester of 596.5 +/- 68.6 g(NH3) L-1. Sterilized mass addition of 10% (w/w) caused intensive foaming, LCFA accumulation of 9172 +/- 701.2 MgCOD-LCFA g(sample)(-1) and termination of the experiment. (C) 2013 Elsevier Ltd. All rights reserved.